The present invention relates to the enhancement of liquid-liquid extraction processes. More particularly, the present invention relates to the utilization of super critical fluid extraction to further process the liquid extract from a liquid-liquid separation process. Further processing is also provided in which the liquid solvent and the supercritical solvent are separated and recirculated to their respective reaction vessels.
Liquid-liquid separation processes are characterized in that a liquid solvent is added to various feed stocks which include mixtures of various compounds. The liquid solvent is selected to dissolve certain components of feedstock. The liquid-liquid extraction process results in the production of a liquid extract which has dissolved therein certain components of the feedstock. The component of the feedstock which is left following removal of the liquid extract is referred to as the raffinate. The material that is usually the desired product of this process is typically found in the liquid extract. The task then becomes one of separating the liquid solvent from the components which are dissolved therein. Typically this separation is carried out through processes such as distillation. However, such processes are typically not energy efficient particularly since the thermal energy which is applied to the liquid extract to boil off the liquid solvent is typically not required. In accordance with the present invention the relatively energy inefficient distillation separation of the liquid extract from its solute is performed by supercritical fluid extraction employing a supercritical solvent, such as carbon dioxide. Such a solvent has a particular advantage that it is relatively easily separated from the liquid solvent thereby rendering it possible to return both the supercritical solvent and the liquid solvent to their respective reaction vessels. While the present invention is generally applicable to enhancement of any liquid-liquid separation process, it is particularly described herein with respect to a treatment of residual oil which is effective for the removal of vanadium compounds.
In order to fully appreciate the residual oil treatment process described herein, it is desirable to consider some of the properties of this particular feedstock. In particular, residual oil is the oil that remains after crude oil is distilled. Residual oil is, however, not a uniquely characterizable fuel. The composition of the residual oil is a function of both the crude oil from which is was made and the conditions under which it was processed. Moreover, determination of the chemical composition of the given residual oil is a difficult task, since it is composed of a wide spectrum of compounds. To avoid this problem it is common in the petroleum industry to describe oil fractions by operational definitions. For example, the various fractions obtained in distillation are described by the "still" temperature at which they were drawn. The still temperature is also referred to as the "cut" temperature. Similarly, the various fractions of residual oil are described by their solubility in known solvents. In particular, the asphaltenes are those compounds which are not soluble in pentane.
In particular, residual oil is known to contain quantities of vanadium, which is deleterious to gas turbine power plants employing such residual oil as a fuel. The vanadium is chemically bound in some of the organic molecules in the residual oil, thus is soluble in the oil. Typically, 50% of the vanadium is incorporated into the oil as a porphyrnic structure. The basic porphyrnic structure has a molecular weight of approximately 400, and can exist as a monomer or polymer, or be associated with other forms, and thus can grow into very large structures. Similarly, the non-porphyrnic forms of vanadium span a wide range of molecular weights. It is noted that about 70% of the vanadium is associated with asphaltenic elements in the residual oil. Such elements make up approximately 15% by weight of the oil. Accordingly, it is seen that deasphalting the residual oil with a yield of about 85% results in removal of about 70% of the undesired vanadium. Deasphalting the residual oil may be accomplished by distillation, or as proposed herein, by the utilization of energy efficient liquid-liquid extraction process.
The deleterious effects of vanadium in residual oil as a gas turbine fuel are mitigated by the utilization of magnesium injection. However, magnesium injection results in shorter than desirable maintenance periods. Additionally, it is noted that the magnesium injection required to mitigate the corrosive effects of the vanadium is directly proportional to the vanadium content in the residual oil field. It is therefore seen that one of the objects of the present invention is the removal of this vanadium from residual oil prior to its utilization as a fuel in gas turbine power plants, particularly those used for electrical energy generation. However, it is noted that the present invention is nonetheless applicable to a wide range of processes and feedstocks. In particular, the present invention is applicable to any liquid-liquid extraction process employing a solvent for which there is available a supercritical fluid solvent which may be readily separated from the liquid solvent.
Supercritical fluid extraction (SCFE) is a chemical engineering unit operation whose applications are growing rapidly. As the name implies, supercritical fluid extraction is an extraction process in which a fluid above its critical point is used as the solvent. Accordingly, SCFE is in a sense a hybrid between liquid extraction and distillation in that the fluid has the flow properties of a gas and yet the solvent properties of a liquid. Although the principles of SCFE have been known for over 100 years, the operation has only recently begun to elicit interest as its advantages have only recently been recognized. Some of these advantages are that supercritical fluid extraction is energy efficient, selective, runs at a relatively low temperature and can exhibit a larger solubility difference over a narrower range of conditions, than either distillation or liquid-liquid extraction. SCFE has in fact also been used for deasphalting oils.